In many of today's ever more challenging laser applications there is a continued quest for shorter pulses which carry high energies per pulse. These features promise better control and greater operating speed for laser applications. A notable step in the evolution of the field was the appearance and maturation of laser systems outputting ultra-short, femtosecond laser pulses. These femtosecond lasers can be used for a wide variety of applications, including several different types of ophthalmic surgeries, where the ultra-short pulses can be used to modify the targeted ocular tissue in a well-controlled manner.
In early femtosecond lasers the extreme shortness of the pulse length lead to an extreme high power in these pulses. This high power, however, threatened to damage the gain medium of the lasers. The solution arrived in the form of the chirped pulse amplification (CPA) technique. In the CPA technique femtosecond seed pulses are generated by an oscillator or seed laser. The seed pulses are directed to a stretcher that stretches the length of the seed pulses by a factor of 10-1,000 to the picosecond range, thus drastically reducing the power within a pulse. These stretched pulses can be safely amplified with the gain medium of the amplifier without damaging the gain medium itself. The amplified pulses then are sent to a compressor that compresses the length of the amplified pulses back to femtoseconds. Lasers based on the CPA approach have been introduced into a large number of applications successfully up to date.
The performance of CPA systems is very sensitive to the stretcher performing the stretching without undermining the beam quality and the compressor being precisely tuned to the stretcher to be able to compress the pulses with high efficiency. Without these performance factors being just right, the compression of the laser pulses becomes incomplete and the length of the pulses does not get compressed back to the desired femtosecond range. Therefore, the fine tuning of the stretching and compression in chirped pulse amplification lasers remains a challenge.